The gas atomization of melting alloy is a main method for superalloy powder preparation. However, a main problem raised in such a method is that a large amount of prepared powder may contain closed pores filled with atomizing gas, which is defined as hollow powder. The hollow defects in power is completely sealed, which is difficult to be eliminated in subsequent powder-forming process. Thus, the hollow defects will remain in the materials and finally form pores. In the meanwhile, residual gas sealed in hollow defects will expand during subsequent heat-treatment and service. All of those factors lead to the formation of heat-induced pore, or heat-induced crack, which severely deteriorates materials mechanical properties, especially for powder metallurgy superalloy. Therefore, hollow powder is one of the main sources of those defects, and severely deteriorates superalloy mechanical properties.
Currently, among superalloy powder prepared through gas atomization process, the ratio of hollow powder to solid powder in those of particle size over 75 μm (200 meshes) is relative high, and ratio of hollow powder to solid powder in small particles is relative low. A method of sieving powder has been applied to remove hollow powder for a long time. In countries such as America and Russia, atomized powder of which particle size is less than or equal to 53 μm (−270 meshes) or 45 μm (−325 meshes) is generally used to prepare superalloy to reduce adverse impact of the powder hollow defect on alloy mechanical properties, but it will cause lower powder utilization efficiency and higher cost. By using the sieving method, large-size hollow powder can be removed, but hollow powder can also generated from undersize particles, which makes the eliminating process incomplete. Moreover, sieving method to remove hollow powder usually suffers low powder utilization efficiency, serious waste and increased cost of alloy preparation.
With regard to the problem of hollow defects in atomized powder during powder preparation, controlling atomization process parameters is a main method to reduce the hollow ratio of powder. For powder preparation through plasma rotating electrode process (PREP), controlling the rotating speed of electrode bar and pressure of atomized gas are mainly methods to reduce the hollow ratio of powder. When the rotating speed of electrode bar is reduced, the quantity of hollow powder is also reduced, but the content ratio of large-size powder is increased, yield of fines is low, and the hollow size is correspondingly enlarged. When the rotating speed of electrode bar is increased, the quantity of hollow powder is increased, but the yield of fines is high. When the atomized gas pressure is reduced, the quantity of hollow powder is also reduced, but the content ratio of large-size powder is high, and the yield of fines is low. With reducing atomized gas pressure, the melt solidification rate is also reduced. Consequently, the microstructure of solidified powder becomes bulky. For superalloy powder fabricated by argon atomization (AA), detailed process for eliminating powder hollow defects has not been reported, and features of gas atomization technique cause that controlling parameters during the atomization process can only reduce hollow powder ratio, not completely eliminate the powder hollow defect.
So far the method for eliminating hollow defects in atomized powder has not been reported.